The invention relates generally to positron emission tomography (PET) and more particularly to the coincidence timing calibration of PET systems.
Positrons are antimatter electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances which are introduced into the patient and become involved in such processes as glucose metabolism, fatty acid metabolism and protein synthesis.
As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, a matter-antimatter annihilation converts them into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners. Each gamma ray has an energy of 511 keV and the two gamma rays are directed in substantially opposite directions. An image is created by determining the number of such annihilation events at each location within the field of view.
The PET imaging system, or scanner, includes one or more rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Each detector is typically comprised of thirty-six (36) crystals, arranged in a 6×6 array. Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes, millions of events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. This data is used to reconstruct an image.
One important calibration procedure in a PET scanner is the coincidence timing calibration (“CTC”). The purpose of CTC is to correct for relative timing differences in the detectors and the front end electronics of the PET scanner. Some PET scanners utilize a CTC method that calculates a time adjustment value for each detector within the detector rings of the PET scanner. This method is described in U.S. Pat. No. 5,272,344, entitled “Automated Coincidence Timing Calibration For A PET Scanner.” However, timing variations still exist among the crystals within each detector, owing to various factors, including non-uniformity of the PMT cathode response, and different electronic circuits and components through which signals coming from different anodes pass. Since when and where the coincidence events takes place is identified with the crystal, not only the detector, the accuracy of the crystal timing directly affects the ability of accepting true coincidence events and rejecting random events. Therefore, there is a need for an accurate and efficient method for determining a crystal adjustment value for each crystal within each detector located in the PET scanner detector rings.
The invention provides systems and methods for coincidence timing calibration of PET systems, that overcome the disadvantages of known systems and methods.